1. Field of the Invention
The present invention relates to a brushless DC single-phase motor ideally used as a fan motor for exhausting the heat generated in a cabinet of electronic equipment to the outside. More particularly, the present invention relates to a pre-drive circuit for applying control signals to a switching device of a drive circuit of the brushless DC single-phase motor.
2. Description of the Related Art
In electronic equipment, including office automation equipment, typically represented by a personal computer and copying machine, that accommodates a number of electronic parts in a relatively small cabinet, the heat generated from the electronic parts is confined in the cabinet, leading to a danger of the electronic parts being thermally damaged.
To solve such a problem, a vent hole is provided in a wall surface or a ceiling surface of the cabinet of such electronic equipment, and a fan motor is installed at the vent hole so as to exhaust the heat in the cabinet to the outside.
For such a fan motor, a brushless DC single-phase motor is used frequently. A conventional pre-drive circuit for the brushless DC single-phase motor will be described with reference to FIG. 3.
Referring to FIG. 3, the pre-drive circuit is defined by the section of the brushless DC single-phase motor excluding a coil (motor coil) L1 and a drive circuit 31 therefor. Vcc denotes a DC power source for operating the circuit.
As shown in the drawing, the drive circuit 31 is constituted by four switching devices, namely, n-channel MOS power field-effect transistors (FETs) PF1 through PF4, a diode D31, and a capacitor C31.
The coil L1 is provided on a stator (not shown) of the motor, and energized at a predetermined ON/OFF timing by the four power FETs PF1 through PF4 of the drive circuit 31, which drives the coil L1, so as to produce a dynamic magnetic field or a rotating magnetic field.
A rotor (not shown) of the motor is provided with a permanent magnet, and rotated as the permanent magnet rotates, following the magnetic field.
The pre-drive circuit is constructed of dedicated integrated circuits IC1 and IC2, resistors R31 through R35, capacitors C32 through C35, and diodes D32 through D35. The power FETs, PF1 through PF4, have parasitic diodes, as shown in the drawing.
In the following descriptions, the dedicated integrated circuits IC1 and IC2 will be referred to simply as the dedicated IC1 and IC2, and the power FETs, PF1, PF2, PF3, and PF4, will be referred to simply as PF1, PF2, PF3, and PF4.
The dedicated IC1 receives a rotational position signal x of a motor, i.e., a rotor or permanent magnet, detected by a Hall element or the like (not shown), a high-level signal y for shutdown, and a duty ratio setting signal z for controlling the rotational speed of the motor. The dedicated IC1 is subjected to a step-up voltage VB1, which will be discussed hereinafter, to turn ON or OFF the PF1 and PF3 at timings set on the basis of the signals x, y, and z.
The signals x, y, and z are also supplied to the dedicated IC2. Upon receipt of a step-up voltage VB2, which will be discussed hereinafter, the dedicated IC2 turns ON or OFF the PF2 and PF4 at timings set on the basis of the signals x, y, and z.
Of the PF1 through PF4 connected as illustrated, the PF3 and PF4 turn ON if the potentials of their gates, i.e., control input terminals, are slightly higher than a ground potential, because their sources are grounded. The PF1 and PF2 are adjacent to a power source Vcc with the coil L1 installed therebetween. Hence, in a normal mode wherein the drive voltage of the coil L1 is substantially equal to a power supply voltage (Vcc), a voltage exceeding the power supply voltage must be applied to their gates. In other words, a voltage obtained by adding the gate-source voltage required for turning the PF1 and PF2 ON to the power supply voltage must be applied to the gates.
Capturing such voltage higher than the power supply voltage from outside inevitably adds to the complication and size of a power supply circuit, as well as higher cost. For this reason, it is usually desired to obtain such a voltage within the pre-drive circuit itself.
As a solution to the above problem, a step-up circuit, such as a charge pump circuit, is added. Each of the circuit constituted by the diode D32, the capacitor C34, and the resistor R31, and the circuit constituted by a diode D33, the capacitor C35, and the resistor R31 makes up the charge pump circuit.
In this case, a step-up voltage VB1 from a connection point of the diode D32 and the capacitor C34 is applied to the dedicated IC1 as a step-up voltage VB for turning the PF1 ON. Similarly, a step-up voltage VB2 from a connection point of the diode D33 and the capacitor C35 is applied to the dedicated IC2 as a step-up voltage VB for turning the PF2 ON.
Thus, the dedicated IC1 supplies a high-voltage pulse signal HO based on the voltage VB to the gate of the PF1 at a predetermined ON/OFF timing. Similarly, the dedicated IC2 supplies the high-voltage pulse signal HO based on the voltage VB to the gate of the PF2 at a predetermined ON/OFF timing. On the other hand, low-voltage pulse signals LO based on the power supply voltage (Vcc) are supplied from the dedicated IC1 and IC2 to the gates of the PF3 and PF4 at predetermined ON/OFF timings.
The ON/OFF timings are set in the dedicated IC1 and IC2 on the basis of the signals x, y, and z. The signals from the dedicated IC1 and IC2 cause the PF1 through PF4 to turn ON or OFF at predetermined timings and duty ratios so as to control the energization of the coil L1.
Thus, the motor, i.e., the rotor, rotates in a predetermined direction at a speed based on the signals x, y, and z. If the motor is equipped with a fan and installed at the vent hole in the cabinet of electronic equipment, then the motor operates as a fan motor to exhaust the heat in the cabinet to the outside.
The conventional circuit, however, uses the dedicated IC1 and IC2, which are costly.
Furthermore, the use of the dedicated IC1 and IC2 restricts the gate voltages of the PF1 and PF2 adjacent to the power source to the values within a fixed range.
To be more specific, as mentioned above, the gate voltages of the PF1 and PF2 must be higher than the power supply voltage. The voltages are obtained by a charge pump circuit or the like, and applied to the gates of the PF1 and PF2 by the dedicated IC1 and IC2 on the basis of the voltages applied as the step-up voltages VB1 and VB2 to the VB terminals, namely, the step-up voltage input terminals, of the dedicated IC1 and IC2. However, the ICs, namely, the dedicated IC1 and IC2, are housed in a single package, and the circuit configuration therein is fixed. This means that the range of the step-up voltages VB1 and VB2 applied to the VB terminals of the PF1 and PF2, i.e., the range wherein the gate voltages of the PF1 and PF2 can be changed, depends on the specifications of the ICs, and therefore cannot be arbitrarily changed.
Hence, the degree of freedom in the circuit design involving the ICs is unavoidably limited, making it impossible to permit flexible circuit design that involves, for example, a change of a motor to be driven, a change of the rated drive voltage of the motor coil L1, or a change of the power FETs (the PF1 through PF4) themselves, that would cause a change in the gates voltages of the PF1 and PF2. Especially if a change is made to increase the gate voltages (step-up voltages) of the PF1 and PF2, failure to pay attention to the rating of the step-up voltage input terminals when applying the increased voltages would present a problem of damaging the dedicated IC1 and IC2.